Energy in Magnetic Field

[darkar]

I am curious about is there any energy can be found / get in the magnetic field?

[zefram_c]

Yes: in classical electrodynamics, a magnetic field in vacuum has an energy density of u0B2. This energy can be extracted. For example, a conductor moving at an angle through a magnetic field develops a potential difference between its endpoints. This can be used to run a current, thus performing work. The energy for that could only have come from the field, and indeed the resulting assembly produces a magnetic field that partially cancels the original field, maintaining energy conservation.

[darkar]

Then where is the energy come from?

[zefram_c]

Which energy are you asking about exactly? The original energy of the magnetic field? If so, that energy came from the act of setting up the magnetic field in the first place. See this for a simple example:

http://farside.ph.utexas.edu/teaching/302l/lectures/node86.html

[Creator]

Yes: in classical electrodynamics, a magnetic field in vacuum has an energy density of u0B2.

.

Uh...you of course meant to say energy density is B2/2u0, right zefram?

Creator :wink:

[zefram_c]

Indeed that's what I meant. Can't everyone take a nap in class every now and then?